I believe for ATI you can have up to four GPUs from the same series (like HD3000 or HD4000, but not mixed) together. With nVidia you can have four GPUs, but only using two GTX 295 or 9800GX2. You can have three with GTX 280/260, GTS250, 9800GTX vanilla/+, and 8800GTX/Ultra. The rest of nVidia's GPUs can only do SLI with to cards.